This invention relates to infrared radiation imaging devices and methods of manufacturing such imaging devices. The invention also relates to manufacturing detector elements for such imaging devices.
Infrared radiation imaging devices are known comprising infrared radiation detector elements formed in a body of material sensitive to infrared radiation Each imaging device also comprises a substrate, having a major surface on which the body is mounted, and circuit elements for processing signals derived from the detector elements. Each detector element comprises a region of one conductivity type which forms with an adjacent part of the body a p-n junction for detecting charge-carriers generated in the material by the infrared radiation. These regions are electrically connected to the circuit elements via a conductor pattern present at the major surface of the substrate. An example of such an imaging device is disclosed in an article by P. Felix, et al entitled "CCD Readout of Infrared Hybrid Focal-Plane Arrays" (I.E.E.E. Transactions on Electron Devices, Vol. ED-27, No. 1, Jan. 1980 pages 175-188).
As described in this article such a device structure is adopted in order to couple an array of photovoltaic detector elements to signal processing circuitry in the substrate. In a commonly desired form, the substrate comprises a silicon charge-coupled device for time-delay and integration (TDI) processing of the signals from the detector elements. By contrast the detector material may be, for example, indium antimonide, lead tin telluride or cadmium mercury telluride. Depending on whether current coupling or voltage coupling is used, the substrate conductor pattern (to which the detector region is connected) may be doped semiconductor regions in the substrate (for example a diffused input zone of the CCD) or may be conductor layers on an insulating layer on the substrate (for example an insulated gate at the CCD input).
As recognized in the Felix et al article, the principal technological difficulty with such hybrid imaging devices concerns the means for electrically connecting each detector element to the corresponding input of the substrate circuit. It is conventional for the detector body to be secured to the substrate by metallization forming an electrical connection to the body. In the form shown in FIG. 10(a) of the article the detector element regions which are connected to the substrate conductor pattern are restricted to mesas at the surface of the detector body facing the circuit substrate. The p-n junctions formed by these regions terminate at the side walls of the mesas containing the regions. The detector body and the circuit substrate are linked together only by contacts in the form of metallization columns of indium which constitute both the mechanical and electrical interface between the regions of the detector elements and the input of the substrate circuit. Apart from these columns, the detector body is separated from the circuit substrate. The radiation which is to be detected by generating charge carriers at the radiation-sensitive p-n junction may be incident at either the back surface of the silicon CCD substrate or the front surface of the detector body. In the first case significant absorption of the radiation may occur in transmission from the CCD substrate through the thick metallization columns in front of the junction.
Therefore, preferably, the radiation is imaged onto the detector body, but in this case the radiation or at least the charge-carriers generated by the radiation must cross the bulk of the body to reach the junctions contained in the mesas on the far side of the detector body. The carriers are generated through the detector body, and the sensitivity of the junctions depends on diffusion of the carriers through the bulk to the junctions. Since the carriers may recombine before reaching the junction, the sensitivity is impaired. Furthermore, transverse diffusion of the carriers results in cross talk between the detector elements. The presence of the metallization in the areas behind the junction may cause scattering of the radiation and contribute to cross talk between the detector elements. The performance of the detector elements can also be degraded by stress induced in these areas by the detector body being secured to the circuit substrate by the metallization.
Furthermore, it can be difficult to assemble the detector body on the circuit substrate in this way in a reliable manufacturing process, particularly if it is desired to maintain a small well-defined area for the connecting metallization. The mesas of the detector body must be carefully aligned with the metallization columns on the substrate conductor pattern, which is technically difficult in a manufacturing process, particularly for a compact closely spaced array. High temperatures are required to reflow and bond the indium to the detector mesas. Such temperatures can degrade the characteristics of the detector elements. Furthermore, with such an imaging device structure it is necessary to fabricate and test the detector elements before mounting the detector body on the circuit substrate. These are expensive steps, so that after carrying them out it is undesirable to use a connection technique which can significantly reduce the yield of satisfactory detectors. Even in a satisfactory detector it appears that the indium can easily fracture due to thermal stress when cooled during operation.
An alternative device arrangement is illustrated in FIGS. 10(b) and 11 of the Felix et al article in which separate detector bodies for each detector element are used instead of mesas on a thicker common body. These bodies can be thin so that radiation incident on the detector bodies is not significantly absorbed before reaching the junction. However, the bodies are still bonded to the substrate by metallizations forming connections to the detector regions facing the substrate, and so they suffer from some of the attendant disadvantages, such as performance degradation by induced strain and difficulties in achieving compact closely-spaced arrays. Cross talk between detector elements is eliminated by having a separate body for each detector element, but this arrangement requires a separate connection for the other region of each of the detector elements since this other region is no longer common. As illustrated in FIGS. 10(b) and 11, these separate connections need to be insulated from the p-n junctions by an insulating layer on the side walls of the detector bodies and their continuation as a metallization pattern between the detector bodies may result in a significant proportion of the area on which the radiation is incident being insensitive. Furthermore, these detector elements are again fabricated and tested before being mounted on the substrate conductor pattern, and bonding of the bodies to the conductor pattern can degrade the detector characteristics. It is also a time-consuming process to position and align individually a large number of separate detector bodies as an assembled array on the circuit substrate for the bonding operation.